The adhesion of layers separated by the PVD (physical vapor deposition) or PECVD (plasma enhanced chemical vapor deposition) processes is due to chemical bondings. Consequently the surfaces to be coated must be free from contaminations on the atomic scale, in order to ensure that the atoms of the layer approach as closely as possible the atoms of the surface of the substrate.
The initial state of a surface to be coated is generally ill-defined. Starting from the undisturbed basic structure of the substrate material, the following layers may be present, frequently even simultaneously:
material changed by the shaping process PA1 a solid layer of contaminations, such as residues of rolling oils, cooling lubricants, ancillary grinding or polishing agents, oxides, oil carbon PA1 a layer of adsorbate, for example, anti-corrosion agent, water PA1 a loose layer of solid contaminations such as, for example, dust. PA1 rough degreasing in CFC (chlorofluoro hydrocarbon) PA1 fine washing with aqueous cleaning agents (alkaline, neutral PA1 flushing with water PA1 drying in CFC PA1 the use of very high-purity water increases the risk of corrosion, more particularly in the case of low-alloyed steels; PA1 the use of alcohol requires on the one hand comprehensive safety measures (e.g., explosion proofing of the installations) and on the other the distillation system for removing the water, more particularly with fairly large quantities of parts to be cleaned. PA1 the environmental compatibility of the fluorinated salts PA1 deficient penetration into crevices PA1 the process is very expensive, due to the extensive preparation installation for regenerating the salts. PA1 treatment in inhibited acids for the removal of oxides PA1 mechanical treatment by blasting with corundum or glass beads PA1 1. A higher degree of ionization is achieved. PA1 2. Oxides which become positively charged in a DC discharge and therefore prevent ion bombardment, can be effectively removed by a HF discharge, since due to the alternating field the positive charging is cancelled out during the next half-wave by electron bombardment. PA1 3. Due to the peculiarities of high frequency plasma, a more uniform treatment can be performed on complicated parts or of batches. PA1 it is highly problematical to re-equip and operate coating installations with a system for the production of high frequency plasmas in this frequency range. It is difficult to screen the scattered radiation; PA1 the sputtering process does not operate selectively--i.e., not only impurities, but also the basic material are attacked; PA1 the sputtering process involves the occurrence of redeposition effects. Since the process produces hardly any gaseous substances, which can therefore be removed from the plant via the vacuum pumps, the surface atoms knocked out by pulse transmission diffuse through the chamber and condense on the walls or substrates. An optimum cleaning effect can therefore be achieved only if the inner surface of the chamber corresponds to a multiple of the substrate surface. However, if the chamber is charged with a large number of workpieces, the surface ratio is frequently the opposite. PA1 1. As universal a cleaning effect as possible for oils, greases, plastics residues, burnt-in, resinified oils, oxides, ancillary grinding and polishing agents, PA1 2. economic to use, PA1 3. environmentally compatible, PA1 4. easy to handle, more particularly on a production line, as automatable as possible, PA1 5. cleaning should be performed in situ in the coating chamber, to avoid recontamination after cleaning and to coat on reactive surfaces, PA1 6. the process must be compatible with the coating technique used, and it must be possible to re-equip existing installations, PA1 7 the substrate surface and the basic material must not be negatively affected by the cleaning process (e.g., roughenings, corrosion), PA1 8. the process must have high efficient penetration into crevices PA1 9. the metal substrate should be exposed on an atomic basis.
The precise structure of the layer depends in detail on the whole prehistory of the workpiece. In the majority of cases in the prior art this ill-defined structure of the surface means that the workpieces must be subjected to a multi-stage complicated preliminary treatment in order to bring them into a clearly defined, coatable state.
The process probably most frequently used at the present time to prepare metal surfaces for coating comprises the following main steps:
Due to the environmentally harmful effect of the CFCs, considerable efforts have been made to replace their use by other agents for rough degreasing and drying. In the meantime, rough degreasing can be performed using alkaline cleaning agents. Attempts are being made to replace CFC in the drying stage by very high-purity water (conductivity 0.05 .mu.s), with subsequent hot air drying, or by alcohol. These techniques have inter alia the following disadvantages:
We are aware that a Soviet process uses chemically inert organic substances solid at room temperature which enclose impurities on cold surfaces in crystallization. Subsequent flushing with solvents releases the crystals with the impurities from the surface of the workpiece.
In this process the following are problematical:
In addition to the basic processes here mentioned, frequently further cleaning steps are required:
A number of impurities also occur which can be eliminated only to some extent; these include, for example, silicon oils, plastics residues, residues of polishing agents.
Basically the aforedescribed cleaning processes use prior to coating a sputtering treatment--i.e., the substrate surfaces are bombarded with high-energy ions, to remove any impurities which still remain or have reappeared following cleaning. Operations are frequently performed using rare gas ions which are produced by DC glow discharge and accelerated in the direction of the substrate surface. In one variant metal ions are used instead of rare gas ions.
In addition to a DC glow discharge, use is also made of a HF glow discharge at 13.6 MHZ. The advantages of a HF discharge as against a DC discharge are stated in the following points:
The use of HF plasmas with a frequency of 13.6 MHz is found to be disadvantageous from the following aspects:
An optimum cleaning process for a subsequent PVD or PECVD coating on metal surfaces should meet the following demands: